Generally, in a parallel plate type plasma processing apparatus, a negative DC potential Vdc is generated in an electrode or a high frequency electrode to which a high frequency (RF) for generating a plasma is applied. From this, in a parallel plate type plasma etching apparatus, a high frequency is conventionally applied to a lower electrode or a susceptor on which a substrate to be processed is mounted, in order for ions in a plasma to be vertically drawn onto a surface of the substrate with the force of an electric field by a negative DC potential Vdc on the surface of the susceptor, thereby performing anisotropic etching or reactive ion etching (RIE). Further, such a DC potential Vdc on the high frequency electrode has a correlation with an etching condition, a high frequency discharge state in a processing vessel or the like. For example, if a gas pressure in the processing vessel is lowered, the absolute value of the DC potential Vdc becomes higher. Moreover, if an extraordinary condition occurs in a high frequency discharge system by, e.g., deterioration as a result of the elapse of time, it is reflected in the DC potential Vdc (generally, the absolute value of Vdc is increased). Thus, the DC potential Vdc has been measured to represent a process parameter indicating a variation of plasma processing conditions or a maintenance parameter indicating a repair or replacement timing of a high frequency component or a member.
Conventionally, a voltage sense line is connected to a high frequency electrode or a power feed rod directly coupled therewith, and a DC potential Vdc detected by the voltage sense line is entered as an analog DC voltage to a voltage measurement circuit, thereby obtaining a measurement value of the DC potential Vdc.
In a latest plasma processing apparatus, the power of a high frequency used tends to be increased in order to enhance the efficiency or miniaturization of a plasma processing; and accordingly, a peak-to-peak value of a high frequency voltage being propagated through the power feed rod or the high frequency electrode is increased. For this reason, in a conventional measurement method for introducing a DC potential Vdc into a voltage measurement circuit by putting a voltage sense line in contact with the high frequency electrode or the power feed rod, a high frequency that originally has to be supplied to a high frequency electrode runs off to the ground from a place in which a voltage measurement unit is installed (being leaked or discharged) through a measurement circuit in the unit or a unit housing. Therefore, the measurement unit itself may be damaged or cause to be damaged, and there are problems that could have negative impact on a high frequency discharge in a processing vessel and a characteristic of a plasma generation and, further, a plasma processing quality.